1. Field of the Invention
This invention relates to an exposure method and apparatus for reproducing a pattern of a mask, for example, for manufacturing semiconductor devices onto a photosensitive surface of a substrate using holographic techniques, and more particularly to an exposure technique for utilizing holographic techniques also for an alignment process for adjusting the relative positional relationship between a hologram on which information corresponding to a mask pattern is recorded and the surface of the substrate.
2. Description of the Prior Art
With respect to a holographic technique useful to reproduce a fine pattern for integrated circuits by exposure onto a resist layer coated on a wafer, the reference can be made to a report entitled "Holography with Total Internally Reflected Light", by K. A. Stetson, Applied Physics Letters, Vol. 11, No. 7, Oct. 1, 1967, pp. 225-226.
Recording and reconstruction of a hologram of the type mentioned are performed in the following manner. First, upon recording operation, a coherent light beam is irradiated upon a mask, and a subject beam transmitted through and diffracted by a mask enters into a recording medium while another coherent reference beam enters into the recording medium from the opposite side to the mask by way of a prism. Then, the reference beam is totally internally reflected from a boundary of the recording medium from the air. Consequently, the incident and totally reflected reference beam and the subject beam interfere with each other within the recording medium to form interference fringes corresponding to the mask pattern in the recording medium and record them as a hologram. Meanwhile, reconstruction of the thus recorded hologram is performed by irradiating a reconstructing beam conjugate with the reference beam into the hologram recording medium. In this instance, if a wafer is disposed at the position of the mask in place of the mask, then a real image (mask pattern) of the hologram is formed on the wafer. For example, N. J. Phillips actually discloses, in U.S. Pat. No. 4,857,425, a process of manufacturing integrated circuits using holographic techniques of the type just described.
A reconstruction image of a hologram according to holographic techniques is formed at a same position as the position of a mask pattern used for formation of the hologram with respect to the hologram. Consequently, a mask used in the hologram formation procedure is removed after the hologram has been formed, and if a wafer is fixed at the same position instead and the hologram is reconstructed in this condition, then a reconstruction hologram image is formed on the wafer (the position of the preceding mask), and consequently, the mask pattern is exposed to the wafer.
Since the reconstruction hologram image of the mask pattern is not reproduced accurately on the wafer unless the relative positional relationship between the wafer and the hologram upon reproduction of the mask pattern is not the same as the relative positional relationship between the mask pattern and the hologram upon formation of the hologram, an exposure operation to the wafer must necessarily be performed after accurate alignment is established between the two relative positional relationships, that is, in terms of a relative distance (gap in a Z direction), in an planar direction (x-y direction), a rotational error, an inclination and so forth. The report of K. A. Stetson mentioned above does not include any description of utilization of holographic techniques for such alignment.
An example of conventional lithography which makes use of holographic techniques is disclosed also in Japanese Patent Laid-Open Application No. 235319/1991. According to the prior art technique, a fine pattern of a hologram is reproduced onto a resist layer formed on a wafer through a projection lens making use of the fact that an aberration produced with a projection lens can be cancelled by applying a holographic technique.
Generally, in order to produce a circuit pattern of a semiconductor or liquid crystal device, a plurality of exposure operations of masks are required for a same wafer, and it is necessary to achieve positioning (x-y alignment) with respect to a pattern formed already on the wafer together with detection of a gap for focusing (relative distance between the hologram and the mask or wafer). It is to be noted that adjustment of any relative positional relationship such as x-y alignment or gap alignment will be hereinafter referred to "alignment".
In the conventional alignment system disclosed in Japanese Patent Laid-Open No. 235319/1991 mentioned above, an ordinary alignment mark of the shape of a grating is formed on the outside of a fine pattern of a hologram, and an enlarged projection image of the grating mark is formed on a wafer through the projection lens and an overlapping condition between the enlarged projection image and a wafer alignment mark (also having the shape of a similar grating) formed in advance on the wafer is detected. In this instance, since the projection image of the mask arignment mark is formed on the surface of the wafer by way of the projection optical system in order to effect detection of the relative position from overlapping with the wafer alignment mark, an influence of an aberration of the projection optical system, which is disadvantageous for adjustment of the relative positional relationship between the hologram recording medium and the wafer, still remains. Consequently, although the advantage that the influence of an aberration upon reconstruction of a hologram fine pattern is reduced can be obtained, there is a disadvantage that exposure of a fine pattern is difficult from a limitation in accuracy in alignment particularly upon overlapping exposure which involves a plurality of exposure operations. Further, with the alignment system, since the projection image of the mask alignment mark is enlarged by the projection optical system, an observation optical system of a large size is necessitated and also the restriction of the entire apparatus on the arrangement construction is great. Particularly where total internal reflection holography is utilized, since a prism is disposed in contact with the hologram recording medium, the observation optical system cannot be positioned near to the wafer due to the presence of the prism, and it is difficult to increase the resolution of the observation optical system for the mask alignment mark image projected in an enlarged scale. In addition, when it is tried to make use of the present alignment system for total internal reflection holography, a large projection area for the mask alignment mark must be prepared on the surface of the wafer, resulting in reduction of the effective area for a circuit pattern to be reproduced originally onto the wafer.
Further, it is recited in "Development of Lithography Making Use of Holography", Solid State Technology Japanese Version, November, 1991, that a gap between a hologram and a wafer in lithography making use of total internal reflection holography was measured with a multiplexed wavelength phase stepping scanning interferometer by A. Omar Basil et al. In the measurement disclosed in the document, as shown in FIG. 26 attached herewith, a laser beam is introduced perpendicularly into a hologram recording medium 1201 and a wafer 1205, and interference waves between reflection light from the surface of the hologram recording medium 1201 and internal reflection light from the surface of a resist applied to the wafer 1205 are detected by a detector 1208. Then, a gap between the hologram recording medium and the wafer is measured from a variation of the detector output which arises from the gap. The gap measurement system disclosed in the document is disadvantageous in that, since a laser beam (parallel beam) is introduced perpendicularly into both of the wafer and the hologram recording medium, the result of the measurement is liable to be influenced by multiple reflection which occurs between the two elements. Further, since it is necessary to measure the gap upon hologram recording in advance and then adjust the gap to the measurement value upon reconstruction, accurate measurement of the gap is repeated upon recording and reconstruction of the hologram, resulting in reduction of the throughput of the exposure apparatus.
It cannot be denied that, up to the present, a hologram is sometimes deformed in such operations as recording, fixing and reconstruction of the hologram. For example, when such deformation as partial contraction of a medium on which a hologram is recorded occurs, also the position at which a reconstruction image of the hologram is formed is varied by the thus deformed hologram. Consequently, at the exposure step to a wafer, the position of the wafer must necessarily be modified from the position measured precedently upon recording of the hologram. However, any of the prior art techniques can little achieve alignment for coping with such hologram deformation particularly including correction against a variation of the focus position.